BUCK converter using low side N-Channel MOSFET

Published by MKDas on

When it comes to mind to use a DC/DC converter the first converter that comes to mind is the BUCK converter. There are lots of designs online as well as dedicated ICs for this purpose. But most of these are high side P-MOS based. In high-side P-MOS-based circuits, the gate driving circuit is a little complex. But if N-Channel MOSFET would be used on the low side, it would be easier than that. In this article, we’ll see how we can design a Low side N-Channel-based Buck converter. Which is actually gives us an opportunity to carry more current as well as simple circuitry. So let’s start!

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What is a buck-converter?

The topic is very popular. When the output voltage is lower than the input voltage of a DC to DC converter we can call it a buck converter in the simple method. There are other types of buck converters. For basics, you can read this article.

The basic circuit of a buck converter is:

Here a switch is used to bypass the power to the load through the inductor coil for a short period of time. Then the switch is kept off for another short period of time. So when the switch is on, the current flows through the coil to the load which is shown in the first circuit. And when the switch is off, the freewheeling diode directs the current which was actually stored in the inductor, and that current path is shown in the second image.

You can imagine the coil like a spring. The spring stores energy when it is pushed then it pushes it (or pull) further to the other end. But even after giving a further push, it pushes some more using the stored energy. Like the spring, the inductor also stores energy as a magnetic field. Which is later released to the circuit.

So it is not a rocket science but for more information, you can read this too.

So what’s the problem with the buck converter?

No, there is no problem with it but there is some limitation of selecting components for a basic buck converter circuit like this one:

The problem is the MOSFET and the associated circuits. The basic buck converter uses P-Channel MOSFET. Which is expensive than N-Channel as well as the current rating is pretty low than the same sized N-Channel. Now if you want to use N-Channel rather than the P-Channel MOSFET then another problem comes, the gate driving circuit. Keeping the N-Channel MOSFET on the high side, it is not easy to use a simple gate driving circuit rather than using dedicated gate driving ICs. But if you compare the prices vs current carrying capacity, you have to change this circuit configuration to Low-side switching.

What’s the benefit of using a Low-side N-channel MOSFET?

Just look at this circuit first:

Is this configuration tells you something? Look carefully, we know for a MOSFET the Gate voltage should be around 10V from the Source. As the source is connected to the GND, can you use a simple Opto-coupler for the gate driving purpose? If you have little experience in electronics and micro-controller, it’s now crystal clear to you.

The benefits are:

  • Simple gate driving circuit required
  • More current capacity can be increased easily
  • Low price than using P-MOS or dedicated gate driving ICs

So how it works?

When the MOSFET is in on condition the current flows through this way:

Besides turning the load on, the inductor is also energized. Which is later released when the MOSFET is in off position and the current flows through this way:

This way the conversion process is done. Depending on the on-time and off-time i.e. the duty cycle, the power delivered to the load is controlled. More the duty cycle, more power to the load. Is it complex? I don’t think so.

So what’s the basic circuit diagram?

This is the circuit, of course you have to use suitable resistors where it should be. And the clock source? You can use any type of like MCUs or analog IC.

Conclusion:

In this article, we have seen how we can use N-Channel MOSFET in our buck converter circuit also use a very simple gate driving circuit. I think this will help you a lot. Thank you for your time, see you soon in the next article where we can use this circuit for practical purposes.

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MKDas

Mithun K. Das; B.Sc. in EEE from KUET. Blog: https://labprojectsbd.com

2 Comments

Asimiyu · May 4, 2021 at 1:33 pm

Very good sir. Thank you for the expected SMPS power supply. The details is cleared. Please, we are expecting AC/DC flybsck or SMPS and DC/DC buck converter. God bless you.

    MKDas · May 4, 2021 at 1:41 pm

    Thank you. I’ll try to post in the future.

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